加入分组列表，以及每个分组记录的中值

Question

我正在尝试将数据连接在一起，我的目标是拥有卖家列表，并在每个卖家旁边查看他出售商品所花费的中位数时间。

以下查询在单个卖家 (dan) 上按预期工作，并返回一个值：

SELECT 
  AVG(timetook) AS Median
FROM (
  SELECT 
    TIMESTAMPDIFF(MINUTE, starttime, endtime) AS timetook,
    @rownum:=@rownum+1 as `row_number`,
    @total_rows:=@rownum
  FROM items, (SELECT @rownum:=0) r
  WHERE total_price > 100 AND seller_name = "dan"
  ORDER BY timetook ASC
) AS temp
WHERE 
  `row_number` = FLOOR((@total_rows + 1) / 2) OR
  `row_number` = CEIL((@total_rows + 1) / 2)
  

但是，当试图将其视为卖家列表时（为了测试，我只对两个卖家进行测试），我得到了卖家中位数的空值。

SELECT 
  q1.seller_name, 
  q1.records_found,
  q2.median_timetook
FROM (
  SELECT 
    seller_name,
    COUNT(*) AS records_found
  FROM items 
  WHERE 
    total_price > 100 AND 
    TIMESTAMPDIFF(MINUTE, starttime, endtime) < 60 
  GROUP BY seller_name 
  HAVING COUNT(*) >= 3
) AS q1
LEFT JOIN (
  SELECT 
    seller_name,
    AVG(timetook) AS median_timetook
  FROM (
    SELECT 
      seller_name,
      TIMESTAMPDIFF(MINUTE, starttime, endtime) AS timetook,
      @rownum:=@rownum+1 AS row_number,
      @total_rows:=@rownum
    FROM 
      items, 
      (SELECT @rownum:=0) r
    ORDER BY 
      timetook ASC
  ) AS temp
  WHERE 
    row_number = FLOOR((@total_rows + 1) / 2) OR
    row_number = CEIL((@total_rows + 1) / 2)
  GROUP BY 
    seller_name
) AS q2 ON q1.seller_name = q2.seller_name
WHERE
  q1.seller_name IN ('ron', 'dan') 
GROUP BY
  q1.seller_name
ORDER BY 
  q1.records_found DESC

Answer 1

我建议使用窗口函数而不是用户变量；后者已被弃用，并计划在未来的 MySQL 版本中删除。窗口函数更易于使用，尤其是在管理分区时。

这是计算每个卖家中位数的一种方法：

select seller_name, avg(duration) as median_duration
from (
    select i.*,
        row_number() over(partition by seller_name order by duration) rn,
        count(*) over(partition by seller_name) cnt
    from (
        select i.*, 
            timestampdiff(minute, starttime, endtime)) duration 
        from items i
    ) i
) i
where rn in ( floor((cnt + 1) / 2), floor( (cnt + 2) / 2) )
group by seller_name